Effect of Ti De-oxidation on Solidification and Post-solidification Microstructure in Low Carbon High Manganese Steel

Abstract

This paper investigates the effect of de-oxidation inclusions on micro-structure in low-carbon steels. Low carbon (0.07 wt%), high Mn (0.9 wt%) steel in a Al 2 O 3 or MgO crucible was deoxidized by adding either aluminum (0.05 wt%) or titanium (0.05, 0.03 or 0.015 wt%) in a 400 g-scale vacuum furnace, and cast in a Cu mold at cooling rates between 2.0-6.0 K/s.The oxide inclusions were identified as Al 2 O 3 (1-3 mm) in the Al-killed steel and Ti-Al-(Mg)-O (0.3-0.5 mm) in the Ti-killed steel. Oxide inclusion sizes in all the Ti-killed steels were smaller and inclusion densities higher than those in the Al-killed steel.Solidification structure, defined as the density of primary dendrite arms within a defined region was finer with increasing inclusion density and as a result, the solidification structure of the Ti-killed steel was finer than that of the Al-killed steel.A Confocal Scanning Laser Microscope (CSLM) and a Differential Scanning Calorimeter (DSC) were used to study the differences in solid state micro-structural evolution between the Ti-killed, Al-killed and the non de-oxidized samples. The growth of austenite grains were studied under isothermal conditions and it was found that both grain-boundary mobility and final grain size were lower in the Ti-killed sample than for the others. With regards to austenite decomposition, during continuous cooling from a comparable austenite grain structure, the resulting austenite decomposition structure was finer for the Ti-killed sample due to a higher Widmanstätten lath density due to precipitation at particles in addition to grain boundaries.

Micro-structure Refinement in Low Carbon High Manganese Steels through Ti-Deoxidation, Characterization and Effect of Secondary Deoxidation Particles

“…[25][26][27][28] Results are summarized as follows. The following results are obtained by evaluating with the samples in Table 1.…”

“…Finally, the molten de-oxidized steel was poured into a water-cooled copper and stainless steel mold ( Inner size: 60×40×40 mm). The Al killed sample and a sample with no material added for deoxidation in the previous study 25) were used for comparison with the Ti-killed samples investigated in this study.…”

“…Details on the CSLM and its capabilities can be found in the pioneering work by Emi and co-workers, [31][32][33] and the CSLM used in this study can be shown in the previous paper. [25][26][27][28] The sample was heated to 1 873 K at a rate of 10 K/s. after which the molten sample was maintained for 1 minute, and subsequently cooled to room temperature at the aimed cooling rate.…”

“…Oxide particles have been utilized as a heterogeneous nucleation site of the precipitations of ferrite, 21) nitrides 22) and sulfides. 23,24) In the previous papers, [25][26][27][28] we reported that the finer solidified microstructure could be obtained by achieving higher density of small secondary oxide particles of sizes below 1 μ m. Furthermore, the effect of oxygen content and cooling rate during solidification on the secondary oxide particle generation had been elucidated. The small secondary oxide particles were shown to have an effect of pinning the austenite grains and to promote ferrite precipitation.…”

Equilibrium Relationship between the Oxide Compounds in MgO–Al2O3–Ti2O3 and Molten Iron at 1 873 K

“…On the other hand, titanium oxide finely dispersed in steel and complex deoxidation including in titanium is considered to be effective for the above-mentioned purpose. 5) A determination of the equilibrium relationship between the oxide compounds in MgO-Al 2 O 3 -Ti 2 O 3 and molten iron is important to avoid Al 2 O 3 or MgO · Al 2 O 3 formation and for inclusion control. However, our previous work 6) is the only study that has reported on this and the equilibrium relationship between the deoxidation products and the composition of the solute elements in molten iron was investigated at 1 973 K. Therefore, in this study we investigated the equilibrium relationship between the oxide compounds in MgO-Al 2 O 3 -Ti 2 O 3 and molten iron at 1 873 K.…”